Remarks

A string is a sequential collection of characters that is used to represent text. A String object is a sequential collection of System.Char objects that represent a string; a System.Char object corresponds to a UTF-16 code unit. The value of the String object is the content of the sequential collection of System.Char objects, and that value is immutable (that is, it is read-only). For more information about the immutability of strings, see the Immutability and the StringBuilder class section later in this topic. The maximum size of a String object in memory is 2GB, or about 1 billion characters.

Instantiating a String object

By assigning a string literal to a String variable. This is the most commonly used method for creating a string. The following example uses assignment to create several strings. Note that in C#, because the backslash (\) is an escape character, literal backslashes in a string must be escaped or the entire string must be @-quoted.

using namespace System;
void main()
{
String^ string1 = "This is a string created by assignment.";
Console::WriteLine(string1);
String^ string2a = "The path is C:\\PublicDocuments\\Report1.doc";
Console::WriteLine(string2a);
}
// The example displays the following output:
// This is a string created by assignment.
// The path is C:\PublicDocuments\Report1.doc

string string1 = "This is a string created by assignment.";
Console.WriteLine(string1);
string string2a = "The path is C:\\PublicDocuments\\Report1.doc";
Console.WriteLine(string2a);
string string2b = @"The path is C:\PublicDocuments\Report1.doc";
Console.WriteLine(string2b);
// The example displays the following output:
// This is a string created by assignment.
// The path is C:\PublicDocuments\Report1.doc
// The path is C:\PublicDocuments\Report1.doc

Dim string1 As String = "This is a string created by assignment."
Console.WriteLine(string1)
Dim string2 As String = "The path is C:\PublicDocuments\Report1.doc"
Console.WriteLine(string2)
' The example displays the following output:
' This is a string created by assignment.
' The path is C:\PublicDocuments\Report1.doc

By calling a String class constructor. The following example instantiates strings by calling several class constructors. Note that some of the constructors include pointers to character arrays or signed byte arrays as parameters. Visual Basic does not support calls to these constructors. For detailed information about String constructors, see the String constructor summary.

Dim chars() As Char = { "w"c, "o"c, "r"c, "d"c }
' Create a string from a character array.
Dim string1 As New String(chars)
Console.WriteLine(string1)
' Create a string that consists of a character repeated 20 times.
Dim string2 As New String("c"c, 20)
Console.WriteLine(string2)
' The example displays the following output:
' word
' cccccccccccccccccccc

By using the string concatenation operator (+ in C# and & or + in Visual Basic) to create a single string from any combination of String instances and string literals. The following example illustrates the use of the string concatenation operator.

String^ string1 = "Today is " + DateTime::Now.ToString("D") + ".";
Console::WriteLine(string1);
String^ string2 = "This is one sentence. " + "This is a second. ";
string2 += "This is a third sentence.";
Console::WriteLine(string2);
// The example displays output like the following:
// Today is Tuesday, July 06, 2011.
// This is one sentence. This is a second. This is a third sentence.

string string1 = "Today is " + DateTime.Now.ToString("D") + ".";
Console.WriteLine(string1);
string string2 = "This is one sentence. " + "This is a second. ";
string2 += "This is a third sentence.";
Console.WriteLine(string2);
// The example displays output like the following:
// Today is Tuesday, July 06, 2011.
// This is one sentence. This is a second. This is a third sentence.

Dim string1 As String = "Today is " + Date.Now.ToString("D") + "."
Console.WriteLine(string1)
Dim string2 As String = "This is one sentence. " + "This is a second. "
string2 += "This is a third sentence."
Console.WriteLine(string2)
' The example displays output like the following:
' Today is Tuesday, July 06, 2011.
' This is one sentence. This is a second. This is a third sentence.

By retrieving a property or calling a method that returns a string. The following example uses the methods of the String class to extract a substring from a larger string.

By calling a formatting method to convert a value or object to its string representation. The following example uses the composite formatting feature to embed the string representation of two objects into a string.

Char objects and Unicode characters

Each character in a string is defined by a Unicode scalar value, also called a Unicode code point or the ordinal (numeric) value of the Unicode character. Each code point is encoded by using UTF-16 encoding, and the numeric value of each element of the encoding is represented by a Char object.

Note

Note that, because a String instance consists of a sequential collection of UTF-16 code units, it is possible to create a String object that is not a well-formed Unicode string. For example, it is possible to create a string that has a low surrogate without a corresponding high surrogate. Although some methods, such as the methods of encoding and decoding objects in the System.Text namespace, may performs checks to ensure that strings are well-formed, String class members don't ensure that a string is well-formed.

A single Char object usually represents a single code point; that is, the numeric value of the Char equals the code point. For example, the code point for the character "a" is U+0061. However, a code point might require more than one encoded element (more than one Char object). The Unicode standard defines two types of characters that correspond to multiple Char objects: graphemes, and Unicode supplementary code points that correspond to characters in the Unicode supplementary planes.

A grapheme is represented by a base character followed by one or more combining characters. For example, the character ä is represented by a Char object whose code point is U+0061 followed by a Char object whose code point is U+0308. This character can also be defined by a single Char object that has a code point of U+00E4. As the following example shows, a culture-sensitive comparison for equality indicates that these two representations are equal, although an ordinary ordinal comparison does not. However, if the two strings are normalized, an ordinal comparison also indicates that they are equal. (For more information on normalizing strings, see the Normalization section.)

A Unicode supplementary code point (a surrogate pair) is represented by a Char object whose code point is a high surrogate followed by a Char object whose code point is a low surrogate. The code units of high surrogates range from U+D800 to U+DBFF. The code units of low surrogates range from U+DC00 to U+DFFF. Surrogate pairs are used to represent characters in the 16 Unicode supplementary planes. The following example creates a surrogate character and passes it to the Char.IsSurrogatePair(Char, Char) method to determine whether it is a surrogate pair.

Strings and The Unicode Standard

Characters in a string are represented by UTF-16 encoded code units, which correspond to Char values.

Each character in a string has an associated Unicode character category, which is represented in the .NET Framework by the UnicodeCategory enumeration. The category of a character or a surrogate pair can be determined by calling the CharUnicodeInfo.GetUnicodeCategory method.

.NET maintains its own table of characters and their corresponding categories, which ensures that a specific version of a .NET implementation running on different platforms returns identical character category information. The following table lists .NET versions and the versions of the Unicode Standard on which their character categories are based.

In addition, the .NET Framework supports string comparison and sorting based on the Unicode standard. In versions of the .NET Framework through the .NET Framework 4, the .NET Framework maintains its own table of string data. This is also true of versions of the .NET Framework starting with the .NET Framework 4.5 running on Windows 7. Starting with the .NET Framework 4.5 running on Window 8 and later versions of the Windows operating system, the runtime delegates string comparison and sorting operations to the operating system. The following table lists the versions of the .NET Framework and the versions of the Unicode Standard on which character comparison and sorting are based.

Strings and embedded null characters

In the .NET Framework, a String object can include embedded null characters, which count as a part of the string's length. However, in some languages such as C and C++, a null character indicates the end of a string;it is not considered a part of the string and is not counted as part of the string's length. This means that the following common assumptions that C and C++ programmers or libraries written in C or C++ might make about strings are not necessarily valid when applied to String objects:

The value returned by the strlen or wcslen functions does not necessarily equal String.Length.

The string created by the strcpy_s or wcscpy_s functions is not necessarily identical to the string created by the String.Copy method.

You should ensure that native C and C++ code that instantiates String objects, and code that is passed String objects through platform invoke, don't assume that an embedded null character marks the end of the string.

Embedded null characters in a string are also treated differently when a string is sorted (or compared) and when a string is searched. Null characters are ignored when performing culture-sensitive comparisons between two strings, including comparisons using the invariant culture. They are considered only for ordinal or case-insensitive ordinal comparisons. On the other hand, embedded null characters are always considered when searching a string with methods such as Contains, StartsWith, and IndexOf.

Strings and indexes

An index is the position of a Char object (not a Unicode character) in a String. An index is a zero-based, nonnegative number that starts from the first position in the string, which is index position zero. A number of search methods, such as IndexOf and LastIndexOf, return the index of a character or substring in the string instance.

The Chars[Int32] property lets you access individual Char objects by their index position in the string. Because the Chars[Int32] property is the default property (in Visual Basic) or the indexer (in C#), you can access the individual Char objects in a string by using code such as the following. This code looks for white space or punctuation characters in a string to determine how many words the string contains.

Module Example
Public Sub Main()
Dim s1 As String = "This string consists of a single short sentence."
Dim nWords As Integer = 0
s1 = s1.Trim()
For ctr As Integer = 0 To s1.Length - 1
If Char.IsPunctuation(s1(ctr)) Or Char.IsWhiteSpace(s1(ctr))
nWords += 1
End If
Next
Console.WriteLine("The sentence{2} {0}{2}has {1} words.",
s1, nWords, vbCrLf)
End Sub
End Module
' The example displays the following output:
' The sentence
' This string consists of a single short sentence.
' has 8 words.

Because the String class implements the IEnumerable interface, you can also iterate through the Char objects in a string by using a foreach construct, as the following example shows.

Module Example
Public Sub Main()
Dim s1 As String = "This string consists of a single short sentence."
Dim nWords As Integer = 0
s1 = s1.Trim()
For Each ch In s1
If Char.IsPunctuation(ch) Or Char.IsWhiteSpace(ch) Then
nWords += 1
End If
Next
Console.WriteLine("The sentence{2} {0}{2}has {1} words.",
s1, nWords, vbCrLf)
End Sub
End Module
' The example displays the following output:
' The sentence
' This string consists of a single short sentence.
' has 8 words.

Consecutive index values might not correspond to consecutive Unicode characters, because a Unicode character might be encoded as more than one Char object. In particular, a string may contain multi-character units of text that are formed by a base character followed by one or more combining characters or by surrogate pairs. To work with Unicode characters instead of Char objects, use the System.Globalization.StringInfo and TextElementEnumerator classes. The following example illustrates the difference between code that works with Char objects and code that works with Unicode characters. It compares the number of characters or text elements in each word of a sentence. The string includes two sequences of a base character followed by a combining character.

For more information about working with units of text rather than individual Char values, see the StringInfo class.

Null strings and empty strings

A string that has been declared but has not been assigned a value is null. Attempting to call methods on that string throws a NullReferenceException. A null string is different from an empty string, which is a string whose value is "" or String.Empty. In some cases, passing either a null string or an empty string as an argument in a method call throws an exception. For example, passing a null string to the Int32.Parse method throws an ArgumentNullException, and passing an empty string throws a FormatException. In other cases, a method argument can be either a null string or an empty string. For example, if you are providing an IFormattable implementation for a class, you want to equate both a null string and an empty string with the general ("G") format specifier.

The String class includes the following two convenience methods that enable you to test whether a string is null or empty:

IsNullOrEmpty, which indicates whether a string is either null or is equal to String.Empty. This method eliminates the need to use code such as the following:

if (str == nullptr || str->Equals(String::Empty))

if (str == null || str.Equals(String.Empty))

If str Is Nothing OrElse str.Equals(String.Empty) Then

IsNullOrWhiteSpace, which indicates whether a string is null, equals String.Empty, or consists exclusively of white-space characters. This method eliminates the need to use code such as the following:

The following example uses the IsNullOrEmpty method in the IFormattable.ToString implementation of a custom Temperature class. The method supports the "G", "C", "F", and "K" format strings. If an empty format string or a format string whose value is null is passed to the method, its value is changed to the "G" format string.

Immutability and the StringBuilder class

A String object is called immutable (read-only), because its value cannot be modified after it has been created. Methods that appear to modify a String object actually return a new String object that contains the modification.

Because strings are immutable, string manipulation routines that perform repeated additions or deletions to what appears to be a single string can exact a significant performance penalty. For example, the following code uses a random number generator to create a string with 1000 characters in the range 0x0001 to 0x052F. Although the code appears to use string concatenation to append a new character to the existing string named str, it actually creates a new String object for each concatenation operation.

You can use the StringBuilder class instead of the String class for operations that make multiple changes to the value of a string. Unlike instances of the String class, StringBuilder objects are mutable; when you concatenate, append, or delete substrings from a string, the operations are performed on a single string. When you have finished modifying the value of a StringBuilder object, you can call its StringBuilder.ToString method to convert it to a string. The following example replaces the String used in the previous example to concatenate 1000 random characters in the range to 0x0001 to 0x052F with a StringBuilder object.

Ordinal vs. culture-sensitive operations

Members of the String class perform either ordinal or culture-sensitive (linguistic) operations on a String object. An ordinal operation acts on the numeric value of each Char object. A culture-sensitive operation acts on the value of the String object, and takes culture-specific casing, sorting, formatting, and parsing rules into account. Culture-sensitive operations execute in the context of an explicitly declared culture or the implicit current culture. The two kinds of operations can produce very different results when they are performed on the same string.

The .NET Framework also supports culture-insensitive linguistic string operations by using the invariant culture (CultureInfo.InvariantCulture), which is loosely based on the culture settings of the English language independent of region. Unlike other System.Globalization.CultureInfo settings, the settings of the invariant culture are guaranteed to remain consistent on a single computer, from system to system, and across versions of the .NET Framework. The invariant culture can be seen as a kind of black box that ensures stability of string comparisons and ordering across all cultures.

Important

If your application makes a security decision about a symbolic identifier such as a file name or named pipe, or about persisted data such as the text-based data in an XML file, the operation should use an ordinal comparison instead of a culture-sensitive comparison. This is because a culture-sensitive comparison can yield different results depending on the culture in effect, whereas an ordinal comparison depends solely on the binary value of the compared characters.

Important

Most methods that perform string operations include an overload that has a parameter of type StringComparison, which enables you to specify whether the method performs an ordinal or culture-sensitive operation. In general, you should call this overload to make the intent of your method call clear. For best practices and guidance for using ordinal and culture-sensitive operations on strings, see Best Practices for Using Strings.

Casing

Casing rules determine how to change the capitalization of a Unicode character; for example, from lowercase to uppercase. Often, a casing operation is performed before a string comparison. For example, a string might be converted to uppercase so that it can be compared with another uppercase string. You can convert the characters in a string to lowercase by calling the ToLower or ToLowerInvariant method, and you can convert them to uppercase by calling the ToUpper or ToUpperInvariant method. In addition, you can use the TextInfo.ToTitleCase method to convert a string to title case.

Casing operations can be based on the rules of the current culture, a specified culture, or the invariant culture. Because case mappings can vary depending on the culture used, the result of casing operations can vary based on culture. The actual differences in casing are of three kinds:

Differences in the case mapping of LATIN CAPITAL LETTER I (U+0049), LATIN SMALL LETTER I (U+0069), LATIN CAPITAL LETTER I WITH DOT ABOVE (U+0130), and LATIN SMALL LETTER DOTLESS I (U+0131). In the tr-TR (Turkish (Turkey)) and az-Latn-AZ (Azerbaijan, Latin) cultures, and in the tr, az, and az-Latn neutral cultures, the lowercase equivalent of LATIN CAPITAL LETTER I is LATIN SMALL LETTER DOTLESS I, and the uppercase equivalent of LATIN SMALL LETTER I is LATIN CAPITAL LETTER I WITH DOT ABOVE. In all other cultures, including the invariant culture, LATIN SMALL LETTER I and LATIN CAPITAL LETTER I are lowercase and uppercase equivalents.

The following example demonstrates how a string comparison designed to prevent file system access can fail if it relies on a culture-sensitive casing comparison. (The casing conventions of the invariant culture should have been used.)

Differences in case mappings between the invariant culture and all other cultures. In these cases, using the casing rules of the invariant culture to change a character to uppercase or lowercase returns the same character. For all other cultures, it returns a different character. Some of the affected characters are listed in the following table.

Character

If changed to

Returns

MICRON SIGN (U+00B5)

Uppercase

GREEK CAPITAL LETTER MU (U+-39C)

LATIN CAPITAL LETTER I WITH DOT ABOVE (U+0130)

Lowercase

LATIN SMALL LETTER I (U+0069)

LATIN SMALL LETTER DOTLESS I (U+0131)

Uppercase

LATIN CAPITAL LETTER I (U+0049)

LATIN SMALL LETTER LONG S (U+017F)

Uppercase

LATIN CAPITAL LETTER S (U+0053)

LATIN CAPITAL LETTER D WITH SMALL LETTER Z WITH CARON (U+01C5)

Lowercase

LATIN SMALL LETTER DZ WITH CARON (U+01C6)

COMBINING GREEK YPOGEGRAMMENI (U+0345)

Uppercase

GREEK CAPITAL LETTER IOTA (U+0399)

Differences in case mappings of two-letter mixed-case pairs in the ASCII character range. In most cultures, a two-letter mixed-case pair is equal to the equivalent two-letter uppercase or lowercase pair. This is not true for the following two-letter pairs in the following cultures, because in each case they are compared to a digraph:

Parsing and formatting

Formatting and parsing are inverse operations. Formatting rules determine how to convert a value, such as a date and time or a number, to its string representation, whereas parsing rules determine how to convert a string representation to a value such as a date and time. Both formatting and parsing rules are dependent on cultural conventions. The following example illustrates the ambiguity that can arise when interpreting a culture-specific date string. Without knowing the conventions of the culture that was used to produce a date string, it is not possible to know whether 03/01/2011, 3/1/2011, and 01/03/2011 represent January 3, 2011 or March 1, 2011.

String comparison and sorting

Conventions for comparing and sorting strings vary from culture to culture. For example, the sort order may be based on phonetics or on the visual representation of characters. In East Asian languages, characters are sorted by the stroke and radical of ideographs. Sorting also depends on the order languages and cultures use for the alphabet. For example, the Danish language has an "Æ" character that it sorts after "Z" in the alphabet. In addition, comparisons can be case-sensitive or case-insensitive, and in some cases casing rules also differ by culture. Ordinal comparison, on the other hand, uses the Unicode code points of individual characters in a string when comparing and sorting strings.

A word sort performs a culture-sensitive comparison of strings in which certain nonalphanumeric Unicode characters might have special weights assigned to them. For example, the hyphen (-) might have a very small weight assigned to it so that "coop" and "co-op" appear next to each other in a sorted list. For a list of the String methods that compare two strings using word sort rules, see the String operations by category section.

A string sort also performs a culture-sensitive comparison. It is similar to a word sort, except that there are no special cases, and all nonalphanumeric symbols come before all alphanumeric Unicode characters. Two strings can be compared using string sort rules by calling the CompareInfo.Compare method overloads that have an options parameter that is supplied a value of CompareOptions.StringSort. Note that this is the only method that the .NET Framework provides to compare two strings using string sort rules.

An ordinal sort compares strings based on the numeric value of each Char object in the string. An ordinal comparison is automatically case-sensitive because the lowercase and uppercase versions of a character have different code points. However, if case is not important, you can specify an ordinal comparison that ignores case. This is equivalent to converting the string to uppercase by using the invariant culture and then performing an ordinal comparison on the result. For a list of the String methods that compare two strings using ordinal sort rules, see the String operations by category section.

The following example illustrates the difference between culture-sensitive and ordinal comparison. The example evaluates three strings, "Apple", "Æble", and "AEble", using ordinal comparison and the conventions of the da-DK and en-US cultures (each of which is the default culture at the time the Compare method is called). Because the Danish language treats the character "Æ" as an individual letter and sorts it after "Z" in the alphabet, the string "Æble" is greater than "Apple". However, "Æble" is not considered equivalent to "AEble", so "Æble" is also greater than "AEble". The en-US culture doesn't include the letter"Æ" but treats it as equivalent to "AE", which explains why "Æble" is less than "Apple" but equal to "AEble". Ordinal comparison, on the other hand, considers "Apple" to be less than "Æble", and "Æble" to be greater than "AEble".

Use the following general guidelines to choose an appropriate sorting or string comparison method:

If you want the strings to be ordered based on the user's culture, you should order them based on the conventions of the current culture. If the user's culture changes, the order of sorted strings will also change accordingly. For example, a thesaurus application should always sort words based on the user's culture.

If you want the strings to be ordered based on the conventions of a specific culture, you should order them by supplying a CultureInfo object that represents that culture to a comparison method. For example, in an application designed to teach students a particular language, you want strings to be ordered based on the conventions of one of the cultures that speaks that language.

If you want the order of strings to remain unchanged across cultures, you should order them based on the conventions of the invariant culture or use an ordinal comparison. For example, you would use an ordinal sort to organize the names of files, processes, mutexes, or named pipes.

For a comparison that involves a security decision (such as whether a username is valid), you should always perform an ordinal test for equality by calling an overload of the Equals method.

Note

The culture-sensitive sorting and casing rules used in string comparison depend on the version of the .NET Framework. In the .NET Framework .NET Framework 4.5 running on the Windows 8 operating system, sorting, casing, normalization, and Unicode character information conforms to the Unicode 6.0 standard. On other operating systems, it conforms to the Unicode 5.0 standard.

Ordinarily, you don't call string comparison methods such as Compare directly to determine the sort order of strings. Instead, comparison methods are called by sorting methods such as Array.Sort or List<T>.Sort. The following example performs four different sorting operations (word sort using the current culture, word sort using the invariant culture, ordinal sort, and string sort using the invariant culture) without explicitly calling a string comparison method, although they do specify the type of comparison to use. Note that each type of sort produces a unique ordering of strings in its array.

Internally, the.NET Framework uses sort keys to support culturallysensitive string comparison. Each character in a string is given several categories of sort weights, including alphabetic, case, and diacritic. A sort key, represented by the SortKey class, provides a repository of these weights for a particular string. If your app performs a large number of searching or sorting operations on the same set of strings, you can improve its performance by generating and storing sort keys for all the strings that it uses. When a sort or comparison operation is required, you use the sort keys instead of the strings. For more information, see the SortKey class.

If you don't specify a string comparison convention, sorting methods such as Array.Sort(Array) perform a culture-sensitive, case-sensitive sort on strings. The following example illustrates how changing the current culture affects the order of sorted strings in an array. It creates an array of three strings. First, it sets the System.Threading.Thread.CurrentThread.CurrentCulture property to en-US and calls the Array.Sort(Array) method. The resulting sort order is based on sorting conventions for the English (United States) culture. Next, the example sets the System.Threading.Thread.CurrentThread.CurrentCulture property to da-DK and calls the Array.Sort method again. Notice how the resulting sort order differs from the en-US results because it uses the sorting conventions for Danish (Denmark).

If your primary purpose in comparing strings is to determine whether they are equal, you should call the String.Equals method. Typically, you should use Equals to perform an ordinal comparison. The String.Compare method is intended primarily to sort strings.

String search methods, such as String.StartsWith and String.IndexOf, also can perform culture-sensitive or ordinal string comparisons. The following example illustrates the differences between ordinal and culture-sensitive comparisons using the IndexOf method. A culture-sensitive search in which the current culture is English (United States) considers the substring "oe" to match the ligature "œ". Because a soft hyphen (U+00AD) is a zero-width character, the search treats the soft hyphen as equivalent to Empty and finds a match at the beginning of the string. An ordinal search, on the other hand, does not find a match in either case.

Module Example
Public Sub Main()
' Search for "oe" and "œu" in "œufs" and "oeufs".
Dim s1 As String = "œufs"
Dim s2 As String = "oeufs"
FindInString(s1, "oe", StringComparison.CurrentCulture)
FindInString(s1, "oe", StringComparison.Ordinal)
FindInString(s2, "œu", StringComparison.CurrentCulture)
FindInString(s2, "œu", StringComparison.Ordinal)
Console.WriteLine()
Dim softHyphen As String = ChrW(&h00AD)
Dim s3 As String = "co" + softHyphen + "operative"
FindInString(s3, softHyphen, StringComparison.CurrentCulture)
FindInString(s3, softHyphen, StringComparison.Ordinal)
End Sub
Private Sub FindInString(s As String, substring As String,
options As StringComparison)
Dim result As Integer = s.IndexOf(substring, options)
If result <> -1
Console.WriteLine("'{0}' found in {1} at position {2}",
substring, s, result)
Else
Console.WriteLine("'{0}' not found in {1}",
substring, s)
End If
End Sub
End Module
' The example displays the following output:
' 'oe' found in œufs at position 0
' 'oe' not found in œufs
' 'œu' found in oeufs at position 0
' 'œu' not found in oeufs
'
' '­' found in co­operative at position 0
' '­' found in co­operative at position 2

Searching Strings

String search methods, such as String.StartsWith and String.IndexOf, also can perform culture-sensitive or ordinal string comparisons to determine whether a character or substring is found in a specified string.

The search methods in the String class that search for an individual character, such as the IndexOf method, or one of a set of characters, such as the IndexOfAny method, all perform an ordinal search. To perform a culture-sensitive search for a character, you must call a CompareInfo method such as CompareInfo.IndexOf(String, Char) or CompareInfo.LastIndexOf(String, Char). Note that the results of searching for a character using ordinal and culture-sensitive comparison can be very different. For example, a search for a precomposed Unicode character such as the ligature "Æ" (U+00C6) might match any occurrence of its components in the correct sequence, such as "AE" (U+041U+0045), depending on the culture. The following example illustrates the difference between the String.IndexOf(Char) and CompareInfo.IndexOf(String, Char) methods when searching for an individual character. The ligature "æ" (U+00E6) is found in the string "aerial" when using the conventions of the en-US culture, but not when using the conventions of the da-DK culture or when performing an ordinal comparison.

On the other hand, String class methods that search for a string rather than a character perform a culture-sensitive search if search options are not explicitly specified by a parameter of type StringComparison. The sole exception is Contains, which performs an ordinal search.

Testing for equality

Use the String.Compare method to determine the relationship of two strings in the sort order. Typically, this is a culture-sensitive operation. In contrast, call the String.Equals method to test for equality. Because the test for equality usually compares user input with some known string, such as a valid user name, a password, or a file system path, it is typically an ordinal operation.

The following example illustrates the danger of performing a culture-sensitive comparison for equality when an ordinal one should be used instead. In this case, the intent of the code is to prohibit file system access from URLs that begin with "FILE://" or "file://" by performing a case-insensitive comparison of the beginning of a URL with the string "FILE://". However, if a culture-sensitive comparison is performed using the Turkish (Turkey) culture on a URL that begins with "file://", the comparison for equality fails, because the Turkish uppercase equivalent of the lowercase "i" is "İ" instead of "I". As a result, file system access is inadvertently permitted. On the other hand, if an ordinal comparison is performed, the comparison for equality succeeds, and file system access is denied.

Imports System.Globalization
Imports System.Threading
Module Example
Public Sub Main()
Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture("tr-TR")
Dim filePath As String = "file://c:/notes.txt"
Console.WriteLine("Culture-sensitive test for equality:")
If Not TestForEquality(filePath, StringComparison.CurrentCultureIgnoreCase) Then
Console.WriteLine("Access to {0} is allowed.", filePath)
Else
Console.WriteLine("Access to {0} is not allowed.", filePath)
End If
Console.WriteLine()
Console.WriteLine("Ordinal test for equality:")
If Not TestForEquality(filePath, StringComparison.OrdinalIgnoreCase) Then
Console.WriteLine("Access to {0} is allowed.", filePath)
Else
Console.WriteLine("Access to {0} is not allowed.", filePath)
End If
End Sub
Private Function TestForEquality(str As String, cmp As StringComparison) As Boolean
Dim position As Integer = str.IndexOf("://")
If position < 0 Then Return False
Dim substring As String = str.Substring(0, position)
Return substring.Equals("FILE", cmp)
End Function
End Module
' The example displays the following output:
' Culture-sensitive test for equality:
' Access to file://c:/notes.txt is allowed.
'
' Ordinal test for equality:
' Access to file://c:/notes.txt is not allowed.

Normalization

Some Unicode characters have multiple representations. For example, any of the following code points can represent the letter "ắ":

U+1EAF

U+0103 U+0301

U+0061 U+0306 U+0301

Multiple representations for a single character complicate searching, sorting, matching, and other string operations.

The Unicode standard defines a process called normalization that returns one binary representation of a Unicode character for any of its equivalent binary representations. Normalization can use several algorithms, called normalization forms, that follow different rules. The .NET Framework supports Unicode normalization forms C, D, KC, and KD. When strings have been normalized to the same normalization form, they can be compared by using ordinal comparison.

An ordinal comparison is a binary comparison of the Unicode scalar value of corresponding Char objects in each string. The String class includes a number of methods that can perform an ordinal comparison, including the following:

The following simple example illustrates string normalization. It defines the letter "ố" in three different ways in three different strings, and uses an ordinal comparison for equality to determine that each string differs from the other two strings. It then converts each string to the supported normalization forms, and again performs an ordinal comparison of each string in a specified normalization form. In each case, the second test for equality shows that the strings are equal.

String operations by category

The String class provides members for comparing strings, testing strings for equality, finding characters or substrings in a string, modifying a string, extracting substrings from a string, combining strings, formatting values, copying a string, and normalizing a string.

Comparing strings

You can compare strings to determine their relative position in the sort order by using the following String methods:

Compare returns an integer that indicates the relationship of one string to a second string in the sort order.

CompareOrdinal returns an integer that indicates the relationship of one string to a second string based on a comparison of their code points.

Testing strings for equality

You call the Equals method to determine whether two strings are equal. The instance Equals(String, String, StringComparison) and the static Equals(String, StringComparison) overloads let you specify whether the comparison is culture-sensitive or ordinal, and whether case is considered or ignored. Most tests for equality are ordinal, and comparisons for equality that determine access to a system resource (such as a file system object) should always be ordinal.

Trim removes all occurrences of a character from the beginning and end of a string.

TrimEnd removes all occurrences of a character from the end of a string.

TrimStart removes all occurrences of a character from the beginning of a string.

Important

All string modification methods return a new String object. They don't modify the value of the current instance.

Extracting substrings from a string

The String.Split method separates a single string into multiple strings. Overloads of the method allow you to specify multiple delimiters, to determine the maximum number of substrings that the method extracts, and to determine whether empty strings (which occur when delimiters are adjacent) are included among the returned strings.

Combining strings

Join concatenates one or more substrings into a single element and adds a separator between each substring.

Formatting values

The String.Format method uses the composite formatting feature to replace one or more placeholders in a string with the string representation of some object or value. The Format method is often used to do the following:

To embed the string representation of a numeric value in a string.

To embed the string representation of a date and time value in a string.

To embed the string representation of an enumeration value in a string.

To embed the string representation of some object that supports the IFormattable interface in a string.

To right-justify or left-justify a substring in a field within a larger string.

For detailed information about formatting operations and examples, see the Format overload summary.

Normalizing a string

In Unicode, a single character can have multiple code points. Normalization converts these equivalent characters into the same binary representation. The String.Normalize method performs the normalization, and the String.IsNormalized method determines whether a string is normalized.

For more information and an example, see the Normalization section earlier in this topic.

Initializes a new instance of the String class to the value indicated by a specified pointer to an array of 8-bit signed integers, a starting position within that array, a length, and an Encoding object.

Compares substrings of two specified String objects, ignoring or honoring their case and using culture-specific information to influence the comparison, and returns an integer that indicates their relative position in the sort order.

Compares substrings of two specified String objects using the specified comparison options and culture-specific information to influence the comparison, and returns an integer that indicates the relationship of the two substrings to each other in the sort order.

Compares two specified String objects, ignoring or honoring their case, and using culture-specific information to influence the comparison, and returns an integer that indicates their relative position in the sort order.

Compares two specified String objects using the specified comparison options and culture-specific information to influence the comparison, and returns an integer that indicates the relationship of the two strings to each other in the sort order.

Reports the zero-based index of the first occurrence of the specified character in this instance. The search starts at a specified character position and examines a specified number of character positions.

Reports the zero-based index of the first occurrence of the specified string in this instance. The search starts at a specified character position and examines a specified number of character positions.

Reports the zero-based index of the first occurrence of the specified string in the current String object. Parameters specify the starting search position in the current string, the number of characters in the current string to search, and the type of search to use for the specified string.

Reports the zero-based index of the first occurrence of the specified string in the current String object. Parameters specify the starting search position in the current string and the type of search to use for the specified string.

Reports the zero-based index of the first occurrence in this instance of any character in a specified array of Unicode characters. The search starts at a specified character position and examines a specified number of character positions.

Reports the zero-based index position of the last occurrence of a specified Unicode character within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string.

Reports the zero-based index position of the last occurrence of the specified Unicode character in a substring within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.

Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string.

Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.

Reports the zero-based index position of the last occurrence of a specified string within this instance. The search starts at a specified character position and proceeds backward toward the beginning of the string for the specified number of character positions. A parameter specifies the type of comparison to perform when searching for the specified string.

Reports the zero-based index of the last occurrence of a specified string within the current String object. The search starts at a specified character position and proceeds backward toward the beginning of the string. A parameter specifies the type of comparison to perform when searching for the specified string.

Reports the zero-based index position of the last occurrence in this instance of one or more characters specified in a Unicode array. The search starts at a specified character position and proceeds backward toward the beginning of the string.

Reports the zero-based index position of the last occurrence in this instance of one or more characters specified in a Unicode array. The search starts at a specified character position and proceeds backward toward the beginning of the string for a specified number of character positions.

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. The elements of each group are projected by using a specified function.

Groups the elements of a sequence according to a specified key selector function and creates a result value from each group and its key. Key values are compared by using a specified comparer, and the elements of each group are projected by using a specified function.

Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein. The index of each source element is used in the intermediate projected form of that element.

Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

Returns a filtered collection of elements that contains every element in the source collection, and the ancestors of every element in the source collection. Only elements that have a matching XName are included in the collection.

Returns a filtered collection of elements that contains the descendant elements of every element and document in the source collection. Only elements that have a matching XName are included in the collection.

Returns a filtered collection of elements that contains every element in the source collection, and the descendents of every element in the source collection. Only elements that have a matching XName are included in the collection.